In the chemical industry, multitube reactors are usually used for carrying out catalytic gas-phase reactions over fixed-bed catalysts.
Multitube reactors usually comprise a catalyst tube bundle made up of numerous parallel catalyst tubes and arranged within an outer wall. The catalyst tubes, which usually contain supported or unsupported catalysts, have their open ends fixed so as to form a seal to tube plates and each of them opens into a cap connected to the outer wall at the upper or lower end. Apart from supported catalysts, the catalyst tubes may, alternatively or in addition, contain shell catalysts, solid catalysts or ordered packings of catalyst material arranged like a static mixer. It is also possible to coat the inner surface of the catalyst tubes with catalyst material. The reaction mixture flowing through the catalyst tubes is fed in and discharged via the caps. A heat transfer medium is circulated through the space located between the uppermost and bottommost tube plates and surrounding the catalyst tubes, which space may be divided by deflecting plates, in order to introduce or remove heat of reaction. For this purpose, the outer wall of the multitube reactor has means for feeding in and discharging the heat transfer medium, usually suitable annular inlet and outlet channels through which the heat transfer medium is circulated by means of suitable pumps. After leaving the multitube reactor, the heat transfer medium is again brought to a prescribed temperature, for example in an external heat exchanger, before it re-enters the reactor. As far as exothermic reactions are concerned, hot cooling may be applied for reactor temperature control as well.
The multitube reactors used in industrial production processes have a diameter of several meters. For economic reasons, as large as possible a number of catalyst tubes is used in the reactor. In the case of a reactor having a diameter of several meters, the number of catalyst tubes is usually in the range from 10,000 to 50,000, preferably in the range of 10.000 to 30.000. In the past, it has been considered important to pack the tubes of industrial multitube reactors as tightly as possible in order to achieve as small as possible a reactor diameter for a maximum number of catalyst tubes. Usually, the tubes are positioned in a triangular arrangement, in most cases in an equilateral triangle. A measure used for the compact arrangement of the catalyst tubes is the ratio of the tube spacing t to the external diameter da of a tube. Here, the tube spacing is the distance from the central internal axes of nearest-neighbor catalyst tubes. Known industrial reactors, for instance the reactor described in the examples of DE 44 31 957 A1, have a ratio of tube spacing to external tube diameter of 1.28 or less.
Particularly when carrying out strongly exothermic oxidation reactions, for example the preparation of phthalic anhydride, acrylic acid, methacrylic acid, acrolein, maleic anhydride or glyoxal, precise control of the reaction temperature plays a critical role. In these reactions, a gas mixture is passed through the catalyst tubes which contain a fixed bed of a catalytically active multimetal oxide. For example, multitube reactors are used for preparing phthalic anhydride which is an important intermediate for producing synthetic resins, phthalate plasticizers, phthalocyanine dyes and further fine chemicals. The worldwide production of phthalic anhydride is more than 4,000,000 metric tons per year. Most phthalic anhydride is now produced by gas-phase oxidation of o-xylene using air as oxidant. For this purpose, o-xylene is vaporized, mixed with an excess of air and passed at 340–440° C. over the catalyst present in the catalyst tubes. The catalyst can, for example, comprise a mixture of V2O5 and TiO2 with promoters on ceramic bodies such as porcelain or SiC spheres or rings. Typical dimensions of these ceramic bodies are about 6 mm×6 mm or 8 mm×6 mm, respectively. In this process, the o-xylene is oxidized to phthalic anhydride with a selectivity of 78–80%. With an enthalpy of reaction of about −1110 kJ/mol, the oxidation is strongly exothermic.
Suitable heat transfer media are, in particular, fluid heat transfer media which are liquid in the preferred reaction temperature range from 250° C. to 500° C., preferably from 250° C. to 380° C. For example, the use of melts of salts is particularly useful, such as a melt of a mixture of potassium nitrate, sodium nitrite and sodium nitrate, which melt is particularly preferred in PA synthesis.
The reaction conditions, in particular reactor temperature control, require particular attention for a number of reasons: the large number of tubes in the reactor makes it necessary for the gas mixture flowing into all tubes over the entire cross section to be the same and constant over time, so that the reaction proceeds at the same rate in all tubes and does not proceed particularly quickly in a few preferred tubes. However, the high enthalpy of reaction liberated can, in particular, lead to the catalyst sintering or melting or becoming inactive in individual tubes in the case of deviations from the prescribed temperature range. This is associated with considerable risks for the plant. Inhomogeneities in the throughput also lead to different reaction conditions in the tubes. This results in formation of increased amounts of by-products which reduce the yield and have to be separated from the resulting phthalic anhydride in later purification steps and have to be disposed of. In the gas-phase oxidation, the reaction temperature goes through a maximum along a catalyst tube in the flow direction and this maximum is referred to as a hot spot. Such a hot spot is desirable in principle but problems occur if the hot spot temperature is too high, since this leads both to a reduced catalyst life and to a decrease in the selectivity of the reaction.
In principle, effective reactor temperature control therefore has the task of reducing temperature inhomogeneities over the cross section of the reactor and to prevent the occurrence of undesirably high hot spots.
In the case of previously known reactors, which generally have a very small ratio of tube spacing to external tube diameter, effective reactor temperature control was possible to only a limited extent. Particularly in the case of cylindrical reactor geometries, a transverse stream of the heat transfer medium is passed from a region outside the catalyst tube bundle to an inner space of the reactor which is free of catalyst tubes, or vice versa. This leads to a large pressure drop and thus to a restricted flow of heat transfer medium. In the past, one was therefore forced to use high-performance and consequently very expensive pumping facilities for conveying the heat transfer medium.